This application is a continuation-in-part application Ser. No. 07/590,759 filed Oct. 1, 1990 abandoned. This invention relates to a process for providing an aluminum article of arbitrary shape with a protective coating consisting of first, second and third coatings, referred to herein as "layers", each distinguishable from another, all of which are electrolytically produced. The first layer produced is a conventional porous anodic oxide layer, with open pores visible at as low as 5000.times. magnification, such as has been provided in the prior art by making aluminum the anode in an aqueous strong inorganic acid electrolyte, with a metal or carbon cathode, and sufficient electric current is passed through the cell, so that the aluminum surface is converted to an aluminum oxide layer. Such a porous anodic oxide layer, conventionally produced in an aqueous phosphoric acid, oxalic acid, phosphorous acid, chromic acid, or sulfuric acid electrolyte, is referred to herein as a "conventional anodic oxide layer" or "conventional layer".
The conventional layer is typically produced after electropolishing, or, by etching an aluminum article of arbitrary shape, typically a sheet, in the absence of electric current, then anodizing it in an appropriate electrolyte so that the anodic oxide is integrally formed with the aluminum substrate. Such a conventional layer, which may be as much as 15 mils (381 .mu.m) thick or more, having pores open at their external surface and about as deep, with walls approximately as high as 15 mils, also has a very thin non-porous passivation layer which forms the bottoms of the pores. The passivation layer is sometimes less correctly referred to as a "barrier layer".
The thickness of this passivation layer is a function of anodizing conditions such as the voltage used, and the composition, concentration and temperature of the electrolyte; the higher the concentration, the greater the solubility of the substrate in the electrolyte and therefore the less the thickness of the passivation layer. The rate at which the passivation layer reaches its thickness is typically less than about 13 .ANG./V (Angstroms per volt), so that for a voltage of 40 V the passivation layer is less than about 520 .ANG. thick. The thickness of this layer remains less than about 13 .ANG./V when either the anodizing conditions or the strong acid used, or both, are changed (see "Anodic Oxide Films on Aluminum" by Diggle, J. W. et al). In general, irrespective of the strong acid used and the conditions of the first anodizing step, the passivation layer is less than 0.5 .mu.m (micrometers) thick, typically less than 1000 .ANG..
When the conventional layer is formed with phosphoric acid, this very thin non-porous passivation layer underlying the porous oxide layer has essentially no elemental phosphorus (P) in it, or a very low ratio of P/Al, and has excellent adherence to the substrate. Without the protection of the walls of the porous anodic oxide above it, which walls have a relatively high ratio of P/Al, and provide excellent abrasion resistance depending upon their height and thickness, this non-porous passivation layer, on its own, has limited abrasion resistance because it is so thin, and even more limited chemical resistance. By "non-porous" is meant that there are no pores visible at a magnification of about 10,000.times. or less.
An aluminum substrate protected with only the conventional, thin, first porous anodic oxide layer less than about 10 .mu.m thick offers insufficient protection for the purposes at hand, and economics and other considerations dictate that a thicker first oxide layer is impractical or otherwise undesirable. Therefore the problem to be solved was: how does one enhance the overall protection afforded by a conventional anodic oxide layer about 10 .mu.m thick or more, on an aluminum substrate of arbitrary shape, by providing its surface(s) with at least one additional protective layer, chemically bonded to the anodic oxide surface? Preferably, one should be able to provide the additional protective layer(s) after the conventional layer is formed. The purpose of the protective layer(s) was to provide not only mechanical protection, but also excellent protection against damage by water because the layer is highly hydrophobic, and/or provides high adhesion for a protective coating, preferably by means of a chemical bond rather than mere chain entanglement. For example, an aluminum substrate having a specular reflectance in excess of about 80% was to be maintained despite being exposed to moisture. In another example, an aluminum substrate with a conventional layer about 10 .mu.m thick is to be provided with additional protection against abrasion, and also with a paint. Examples of each of the foregoing are provided in the illustrative examples herein.
This invention more specifically relates to a process, referred to as a "double anodizing process" because the aluminum substrate is twice-anodized, and to the product produced thereby. The process comprises sequentially anodizing the aluminum substrate; initially, the substrate is conventionally anodized in a first electrolytic bath containing an inorganic or carboxylic acid, typically in a phosphoric, sulfuric, or oxalic acid electrolyte, to produce a conventional layer; then, in a second electrolytic bath, the substrate with the conventional layer is anodized in a second electrolytic bath containing an organophosphorus compound. The organophosphorus compound is chosen from a monomeric substituted phosphorous acid referred to herein as an `organophosphonic acid` having at least two carbon atoms in a substituent; or, a substituted monomeric phosphinic acid referred to herein as an `organophosphinic` acid.
Phosphorous acid is also referred to as "phosphonic acid" especially for naming organic compounds, and phosphinic acid is also referred to as `hypophosphorous` or phosphonous acid. These substituted acids are together hereafter referred to herein as "subs/phosphonic/phosphinic acid". Each acid has an organic radical having at least two carbon atoms, which radical may, or may not, have a functional reactive group (referred to as a `leaving group` herein to minimize confusion with a reactive group of an organic coating which may desirably be provided after the substrate is twice-anodized as described herein) for coupling the organophosphorus compound to a reactive organic compound. In addition to a subs/phosphonic/phosphinic acid, a mono- or diester of phosphoric acid results in the formation of a phosphorus-containing ("P-containing") group as a "head" which is chemisorbed onto the conventional layer, and a "tail" which may, or may not, have a `leaving group` (or `reactive group`) for coupling the chemisorbed ester to a reactive organic compound. In contrast to the esters of phosphoric acid, the effectiveness of the esters of subs/phosphonic/phosphinic acid are quite ineffective for the same purpose.
Reaction of the head occurs because in solution, the OH groups of the P-containing electrolyte are mostly dissociated, leaving the O atoms of the OH groups of subs/phosphonic/phosphinic acid to interact with the OH groups available on the surface of the substrate. This interaction appears to be equally true for the phosphoryl O atoms (O which is connected to the P atom with a double bond). Inelastic Electron Tunneling Spectroscopy (ITES) and Surface Enhanced Raman Spectra (SERS) provide evidence which confirms the presence of three O atoms bonded to three Al atoms for each P atom.
Accordingly, this invention relates to the production of a substrate having two additional protective layers, in addition to the conventional porous anodic oxide layer, each of which two layers is simultaneously produced by a single electrolytic processing step. One of the two layers is essentially an organophosphorus monomolecular essentially continuous monolayer ("OMM"), chemisorbed under anodizing conditions, on the exterior surface of the first conventional layer and chemically bonded to its surface, to form a metal oxide-organophosphorus complex; the other (of the two layers) is a barrier layer of non-porous oxide, containing essentially no P, which is generated and progressively grown (as explained in detail herebelow) under the passivation layer, that is, contiguous to the unoxided metal substrate.
The term "chemically bonded" refers to covalent or ionic bonding in which there is a sharing of at least one electron between Al and O, O and P, and, P and C.
For ready recognition, convenience and ease of reference, the conventional anodic porous oxide layer or `conventional layer`, is referred to by the reference symbol "(i)", the OMM, is referred to as "(ii)", and the barrier layer, is referred to as "(iii)". In combination, (i), (ii) and (iii) are referred to as a "triplex coating". In the triplex coating, the layer (i) is sandwiched between OMM (ii) and barrier oxide layer (iii). Each of the surface-bonded molecules in the OMM are in closely-packed essentially contiguous relationship on the surface, the Al atoms of which are bonded to the O atoms bonded to P atoms, in turn bonded to a C atom of the organo-substituent.
In those instances where the OMM (ii) which results from such packing of molecules on the surface is specifically intended to generate a hydrophobic surface, the effectiveness of the OMM is measured by equilibrium Sessile drop water contact angles. Typically, a vapor degreased "as received" 6061-T6 aluminum substrate has a contact angle of from 40.degree.-50.degree.; with a nitric/HF acid etch it has a contact angle of from 12.degree.-18.degree.; with conventional phosphoric acid or sulfuric acid anodizing the contact angle is in the range from 20.degree.-30.degree.; with a dip in octylphosphonic acid the contact angle is 68.degree.; with a dip in perfluorinated phosphonic/phosphinic acids the contact angle is in the range from 84.degree.-92.degree.; by anodizing in perfluorinated phosphonic/phosphinic acids the contact angle is in the range from 103.degree.-109.degree.. Thus with a choice of OMM, the contact angle with the (i) may at least be doubled, and preferably more than tripled.
The unique feature of the triplex coating is that, once the conditions for the first anodizing step are set, the thickness of the conventional layer (i) and its OMM (ii) are essentially constant, being fixed by the conditions of the conventional first anodization. However, the thickness of the OMM (ii) is essentially insensitive to the conditions of the second anodizing step, this thickness being determined by the length of the subs/phosphonic/phosphinic acid, or phosphoric acid ester molecule. Under the conditions of the second anodization, the thickness of the non-porous oxide (iii) can be arbitrarily increased by stepping up the voltage, without affecting the thickness of either (i) or (ii). The triplex coating therefore provides a "tailored" non-porous barrier oxide layer (iii) in coextensive contact overlying the substrate of oxide-free aluminum.
In the art of producing aluminum base sheet which is to be coated with light-sensitive material so that the presensitized base sheet may be used for lithographic printing plates, Berghauser et al had discovered that if a conventionally anodized aluminum sheet was simply dipped in a solution of polyvinyl phosphonic acid, rinsed with water and dried, the polymer filled the pores of the conventional layer and provided an excellent base upon which a light-sensitive coating could be coated. Details of this non-electrolytic process for coating a conventional layer with polyvinyl phosphonic acid are disclosed in U.S. Pat. No. 4,153,461. Because the polymer layer was produced by mechanically dipping the sheet into the solution of polymer, its adhesion to the sheet was mechanical, that is, mainly due to interstitial chain entanglement of polymer chains in the pores of the conventional layer, and, one would expect to be able to improve such mechanical adhesive bonding.
About ten years later, improvement of such adhesion of the polyvinyl phosphonic acid to the aluminum base sheet, was disclosed by Gillich et al, in U.S. Pat. No. 4,448,647 for a process in which they used a "mixed electrolytic bath", namely, one in which they used a mixture of the polyvinyl phosphonic acid (Berghauser et al had used), and a strong inorganic acid such as phosphoric acid. Starting with an etched aluminum substrate which they anodized in this mixed electrolytic bath, they simultaneously anodized the aluminum and sealed its surface (col 1, lines 10-11) producing an aluminum oxide film which showed no porosity of its surface (col 10, line 35), presumably because its pores were filled with the polymer they knew provided a good base for the light-sensitive materials they used for lithographic plate. Further, because they combined the polyvinyl phosphonic acid with the phosphoric acid, the surface they produced showed a high ratio of P/Al in the metal-oxide organic complex surface film. This ratio of P/Al was 0.6 to 0.9:1, and in some instances as high as 2:1 (col 10 lines 14-16).
In their '647 patent, Gillich et al succeeded in solving the problem they set out to solve, and they did so in a single step anodizing process, using the mixed phosphoric and polyvinyl phosphonic acid bath. They failed to make the invention claimed in this application because they did not use a sequential "double-anodizing" process, and, they used a polymeric electrolyte rather than a monomeric one. By a monomeric electrolyte we refer to one in which there are no repeating units of linked hydrocarbyl groups, though there may be up to 10 oxophosphorus groups, preferably no more than 6. Had Gillich et al used a first anodizing step with only the phosphoric acid, and then, in a second step, used a monomeric organophosphonic electrolyte, they would have found that the pores were not filled. They disclosed using monomeric 2-ethylhexyl phosphonic acid, but only in combination with the strong acid, not in a separate electrolytic bath; as a result they made a surface which showed no porosity.
The OMM (ii) is preferably formed with molecules having a bondable O-P linkage and an unreactive tail bonded to the P atom through a C atom, resulting in a chemically resistant layer, resistant both to strong acids and alkalis, unless the molecules of (ii) have a `leaving` group. Whether (ii) has an unreactive or reactive tail, (ii) is also referred to as a "functionalized layer". Though the leaving group is typically to be coupled with the reactive group of a preselected coating, the leaving group may react with the hydroxyl groups available on (i). This might occur, for example, when ethylenediphosphonic acid provides an OMM in which essentially both terminal phosphonic acid groups are chemisorbed on the surface of (i) resulting in a profusion of chains of --CH.sub.2 -- groups protecting the surface of (i) with phosphonic acid groups of closely packed monomeric molecules being essentially contiguous.
To be useful as an aqueous organophosphorus electrolyte, the subs/phosphonic/phosphinic acid, or, phosphoric acid ester, is required to be substantially soluble in water. When the organophosphorus compound is used as an electrolyte deliberately to provide the OMM (ii) with a reactive end group, the OMM (ii) may still be substantially resistant to strong acids and alkalis. Accordingly, (ii) may still be referred to as "chemically resistant".
By "substantially soluble" is meant that the subs/phosphonic/phosphinic acid, or phosphoric acid ester, has sufficient solubility to conduct enough current at a voltage typically used, to anodize the aluminum substrate in an aqueous electrolyte. Such solubility is generally at least 1000 ppm (parts per million by weight of solution) in water, preferably in the range from 1 to 50% by weight, and more preferably from 5 to 25%.
A "functionalized layer" produced on any valve metal is disclosed in U.S. Pat. No. 5,032,237. The term "valve metal" is used generically to refer to aluminum, niobium, tantalum, titanium, tungsten, zirconium and vanadium, each of which is able to form an OMM with an aqueous subs/phosphonic/phosphinic electrolyte, or phosphoric acid ester, to a greater or lesser degree. In the '237 process the functionalized layer was formed in a single-step anodizing procedure, the OMM being formed on the planar non-porous oxide surface as the reaction product of a the subs/phosphonic/phosphinic acid and the oxide.
The basic procedure for forming the functionalized layer of the '237 invention is substantially the same as that for forming the OMM in the instant invention, except that the OMM is now formed on the open-pore surface of the conventional layer (i). The presence of the conventional layer results in the formation of the non-porous barrier oxide layer (iii) under conventional layer (i); more specifically, the layer (iii) is formed under the passivation layer of the conventional layer (i). This layer (iii) is a barrier oxide which is analogously formed as, and has the same very low P/Al ratio as that present, in the barrier oxide layer formed in the '237 process. Recognizing that the formation of the triplex coating will depend upon the reactivity of the P-containing head of the subs/phosphonic/phosphinic acid, or, the phosphoric acid ester, in the electrolyte, with the particular valve metal used, the disclosure of the '237 patent is incorporated by reference thereto as if fully set forth herein.